This invention relates to an aircraft terrain advisory system, in which terrain information is compared with the dynamic flight parameters for that aircraft to indicate the current aircraft trajectory with respect to clearance heights for a terrain to be avoided by the aircraft without exceeding a predetermined acceleration of the aircraft during a pull up manoeuvre. The invention also relates to a method for indicating the current aircraft trajectory with respect to clearance heights for a terrain.
Consider an eye position above the ground, and the various lines of sight radiating from that position. Some of these lines of sight will intercept the ground or objects projecting from the ground, while others will not. The boundary between the lines of sight which do and do not intercept the ground is known as the visual horizon from that eye position
The operation of the aircraft terrain advisory system can be defined analogously. Consider the position of an aircraft above the ground, and consider the possible trajectories emanating from that point. For each of these trajectories it will be either possible or impossible to pull up from that point without exceeding a predetermined acceleration of the aircraft, given the dynamic flight parameters of that aircraft, such that the trajectory avoids hitting the ground. The region defined by trajectories from which pull up can be achieved is the region the current trajectory of the aircraft must remain in to ensure pull up is possible for a given acceleration of the aircraft. In practice the aircraft is required to avoid violating a clearance level above the ground and not just merely the ground.